This invention relates to thermal print heads.
In typical heads, a row of electrically resistive heating elements are defined along the length of a resistive stripe by a series of conductive fingers which cross the stripe at regular intervals. Each heating element is thus spaced apart from the next element by the width of the finger which separates them and each element is bordered by and connected to the two associated fingers which define the element. In order to print a dot on a sheet of heat-sensitive paper, an individual element is heated by driving current through it. A voltage is applied to one of the element's two associated fingers to supply power, and the other finger is grounded to withdraw (sink) power from the element. By printing a set of selected dots in one row, then moving the paper a short distance (in a direction perpendicular to the resistive stripe) to a new position at which another set of selected dots is printed, and then repeating these steps, patterns of dots corresponding to alphanumeric characters or graphic symbols are formed.
Typically, every other finger along the stripe extends in one direction away from the stripe to connect to integrated circuitry which is arranged to permit grounding of various ones of the fingers at different times. The intervening fingers (to which the voltage is to be applied) are not connected to the grounding integrated circuitry, but instead extend away from the stripe in the opposite direction from the grounding fingers. Every other one of these voltage fingers is routed to a first bus which runs parallel to the stripe, while the remaining voltage fingers are routed to a second bus, which also runs parallel to the stripe.
The printing of dots along a row is done in two phases: first, a voltage is applied to the first bus, and appropriate grounding fingers are grounded to cause heating of the elements corresponding to the desired dot positions; second, the voltage is applied to the second bus, and other appropriate grounding fingers are grounded.
Diodes are connected between each voltage finger and the bus to which it is routed to prevent so-called parasitic voltages from appearing on the bus to which the voltage is not being applied. Such parasitic voltages may erroneously cause printing by elements which have not been selected for heating.